Electric motors and their control electronics (converters) are often operated at the limit of their working capacity in order to obtain a maximum of efficiency with the given expenditure (weight, volume, costs). In so doing, it is important not to overheat the electric motor and its converter. If the temperature climbs above a certain limiting value, then either the insulation of the windings of the electric motor or even the power semiconductors in the converter are affected detrimentally. This leads immediately, or perhaps only after a certain time, to the destruction of the motor or its control electronics.
Certain conventional devices make it possible to monitor the temperature of the windings of an electric motor, or the temperature of the power semiconductors in the converter. If the temperature climbs above a limiting value, the output of the motor can be reduced, and damage avoided.
Such temperature monitoring may be performed using a temperature sensor. However, temperature sensors exhibit a certain lag, so that a motor or converter can be destroyed by short, strong current spikes even before a temperature sensor responds. In addition, the temperature sensor does not sit directly on the chips of the power semiconductors, so that their actual barrier-junction temperature cannot be measured.
Certain conventional devices make do without a temperature sensor. For operating an electric motor, often the current flowing in the respective motor phases is measured using a current sensor. If the internal resistance of the motor phases or of the power semiconductors is known, based on the currents measured, it is possible to calculate the power loss in the windings or in the power semiconductors. The specific temperature can then be inferred using a thermal model. By setting a limit for the energy loss produced per time interval, the motor and its control electronics may be protected reliably from overheating.
Such a device is described in greater detail in German Published Patent Application No. 10 2004 022 005. In the model described therein, it is taken into account that during a standstill of the motor or at very low rotational frequencies, the maximum currents allowed in each phase are markedly lower than at higher rotational frequencies. However, the model used can only be implemented easily in those converters which are able to furnish a certain computing power.